1. Field of the Invention
The invention relates to metallized breathable polymer films--polymer films which are gas/vapor permeable and liquid water impermeable--prepared from highly filled polyolefin film and coated with a suitable metal.
2. Related Art
It is known to prepare porous polyolefin films by stretching a precursor film filled with calcium carbonate. "Breathable" films which are gas/vapor permeable and liquid impermeable to aqueous fluids have been described in U.S. Pat. No. 4,472,328, assigned to Mitsubishi Chemical Industries, Ltd. The Mitsubishi patent describes a breathable polyolefin film prepared from a polyolefin/filer composition having from 20 percent to 80 percent by weigh of a filler such as a surface treated calcium carbonate. A liquid or waxy hydrocarbon polymer elastomer such as a hydroxy-terminated liquid polybutadiene was found to produce a precursor film that could be monoaxially or biaxially stretched to make a film breathable. The breathable film described by Mitsubishi is also described in Great Britain Patent No. 2,115,702, assigned to Kao Corporation. The Kao patent further describes a disposable diaper prepared with a breathable film as disclosed by the Mitsubishi patent. The breathable film is used as a backing for the diaper to contain liquid.
U.S. Pat. No. 4,350,655, assigned to Biax Fiber Film, describes a porous polyolefin film containing at least 50 percent by weight of a coated inorganic filler. The precursor film is formed without the addition of an elastomer by employing an inorganic filler surface coated with a fatty acid ester of only silicon or titanium. The precursor film is then rolled between horizontally grooved rollers. Cold stretching of the precursor film at a temperature below 70.degree. C. produces a porous film. Some of the resulting films were stated to be both vapor and liquid permeable, however, at least one film was stated to be permeable to air.
Copending application Ser. No. 024,503 filed Mar. 11, 1987, teaches a breathable film and a process for making said breathable film. For the sake of clarity and continuity, much of the disclosure of this copending application is repeated under the heading "Detailed Description of Preferred Embodiments of the Invention." The instant invention applies a thin metallic coating to the breathable film taught in U.S. Ser. No. 024,503 and produces a metallized film having substantially the same breathable properties as the unmetallized film.
Metallized films possess many properties of great commercial interest. These properties include:
1) Reflection properties which allow the use of metallized films as reflectors to control energy, usually as insulators but also as distributors;
2) Electrical conductivity which helps control static electricity in packages either during packaging or in protecting what is packaged. Electrical conductivity is also important in capacitors and condensers where the property is primary;
3) Brilliance--a "grabber" property when the film is used as a packaging material. This provides a boost for point-of-sale items and is also a property that conveys the aura of quality;
4) Barrier properties--not only a variable ultraviolet and visible light controller but also, in some cases, a gas barrier;
5) The metallized layer is very light in weight because it is so thin--500 Angstroms is considered a fairly heavy coating;
6) The ultrathin coating tends to have better flex resistance than a foil providing the same properties; and
7) Relatively low production cost. If the cost were not attractive, the level of real business activity utilizing metallizing film would not exist.
To obtain the best set of these metallized properties, a judicious choice of film substrate must be made. Oriented polyester films are the most widely used, and at least ten substrates are now being commercially metallized: polyethylene (PE), cast polypropylene (PP), oriented polypropylene (OPP), polyethylene terephthalate (PET), polycarbonate, rigid polyvinyl chloride (PVC), polystyrene, polyimide, polyamide, paper and cellophane. This is not an exhaustive list but rather is intended to show the variety of substrates being metallized.
Oriented polypropylene is being used in capacitors, condensors and transformers. Polyester is being used in roll leaf, stamping foils, condensors, wall coverings, solar control and packaging. Polycarbonate is used in some graphic applications. PVC (only rigid can be used due to plasticizer evaporation during metallizing) is used in flame proof Christmas tinsel and holiday garlands. Polycarbonate, PVC and polystyrene are used in heavy gauges to make thermoformed trays. Polyimide in outer space applications--usually metallized with pure gold. Nylon is finding some limited applications, paper is used in labels and cellophane in wrappers.
Polyethylene and cast polypropylene are two films very familiar to the converting industry but not to the metallizing industry. Relative to polyester and OPP, polyethylene is heat sensitive, extensible and soft so that it must be handled with care. This difference is familiar to the converting industry where polyethylene is routinely printed at high speeds. Although polyethylene is not familiar to the metallized industry, it is being used as a metallized substrate by several metallizers on a commercial basis.
Metalized polyethylene is bright; it provides a rich lustrous metallic finish similar to the brightest of foils, although not as bright as the finish obtained with metallized polyester.
Polyethylene is much softer than the oriented films commonly used. This softness contributes to greater flexibility, and in textile substitute applications, less crinkle noise and a more pleasing hand. Polyethylene is heat sealable and thus can be formed into bags or pouches or it can be laminated to another substrate that has been reverse printed. The combination of brilliant protected graphics and heat sealability can therefore be combined in a single web. Metallized polyethylene is vacuum formable, and when laminated to nylon can constitute the formable web of a meat package.
The low infrared absorption characteristics of polyethylene make it an excellent reflector substrate. Body heat emitted as infra-red (IR) radiation is reflected and returned by a metallized PE film instead of being absorbed where it can be conducted or convected away. This is particularly true of IR radiation in the frequencies associated with normal body temperature.
Embossed polyethylene makes available various surfaces that when metallized create an attractive visual impact. This visual quality renders it suitable for use in many commercial applications.
Thus, metallized films, are already widely used. However, the invention's added feature of breathability lends even greater utility to the metallized film. For instance, a metallized breathable polyolefin film provides a very effective, light weight blanket for camping. Such a film could also be used as an attractive outerskin, in combination with a thin insulating material, in, for example, ski outfits and other active outerwear in cold climates.
While metallized films are both useful and attractive, it is difficult to obtain good adhesion of a metal to a relatively non-polar substrate such as a polyolefin. High electrostatic treatment levels are necessary. Indeed, ten years ago industry experts generally believed that LDPE could not be metallized with good metal adhesion.
When the breathable film of the parent application was metallized, much better than expected adhesion was obtained. While not wishing to be bound by any theory, it is speculated that the better than expected adhesion may be due to the larger surface area exposed by the breathable pore spaces and/or the increased polarity of the breathable film caused by the fillers in its composition. Of course, the good metal adhesion and substantial retention of breathability properties may be due to some other phenomenon.